export: Display message about writing file

This makes it consistent with Foreman (See #109) and is just nice for the user to know what is happening. Looks like this:

```
[marca@marca-mac2 export_test]$ honcho export supervisord .
[honcho export] writing: export_test.conf

[marca@marca-mac2 export_test]$ honcho export upstart .
[honcho export] writing: export_test-ansvc.conf
[honcho export] writing: export_test-ansvc-1.conf
[honcho export] writing: export_test.conf
```